A component which is built in an electronic device such as a mobile device is required to be reduced in size and weight. For example, a filter used in a mobile device is required to allow precise adjustment of frequency characteristics as well as to be miniaturized. As one of filters which satisfy these requirements, a filter using a thin-film elastic wave resonator has been known (refer to patent document 1).
Hereinafter, with reference to FIG. 23, a conventional thin film elastic wave resonator will be described.
FIG. 23(a) is a diagram illustrating a cross-sectional view of a basic structure of the conventional thin-film elastic wave resonator 500. The thin film elastic wave resonator 500 has a structure in which a piezoelectric body 501 is sandwiched between an upper electrode section 502 and a lower electrode section 503. This thin film elastic wave resonator 500 is used being placed on a semiconductor substrate 505 having a cavity 504 formed therein. The cavity 504 can be formed through partially etching a back face of the semiconductor substrate 505 by using a fine processing technology. In this thin film elastic wave resonator 500, an electric field is applied by the upper electrode section 502 and the lower electrode section 503 in a thickness direction and vibration in the thickness direction is generated. Next, operations of the thin film elastic wave resonator 500 will be described with reference to longitudinal vibration in a thickness direction of an infinite flat plate.
FIG. 23(b) is a schematic diagram illustrating an oblique perspective view for describing the operations of the conventional thin film elastic wave resonator 500. When in the thin film elastic wave resonator 500, the electric field is applied between the upper electrode section 502 and the lower electrode section 503, electric energy is converted to mechanical energy by the piezoelectric body 501. The induced mechanical vibration is vibration expanding in the thickness direction, and expands and contracts in the same direction as that of the electric field. In general, the thin film elastic wave resonator 500 utilizes resonant vibration of the piezoelectric body 501 in the thickness direction and operates with resonance of a frequency at which a thickness of the piezoelectric body 501 is equal to a half-wave length. The cavity 504 shown in FIG. 23(a) is utilized in order to secure the longitudinal vibration in the thickness direction of the piezoelectric body 501.
An equivalent circuit of the thin film elastic wave resonator 500, as shown in FIG. 23(d), has both series resonance and parallel resonance. This equivalent circuit comprises: a series resonance section having a capacitor C1, an inductor L1, and a resistor R1; and a capacitor C0 connected in parallel with the series resonance section. In this circuit configuration, admittance frequency characteristics of the equivalent circuit are, as shown in FIG. 23(c), that an admittance is maximum at a resonance frequency fr and minimum at an anti-resonance frequency fa. Here, a relationship between the resonance frequency fr and the anti-resonance frequency fa is as follows.fr=1/{2π√{square root over ( )}(L1×C1)}fa=fr√{square root over ( )}(1+C1/C0)
When the thin film elastic wave resonator 500 having such admittance frequency characteristics is adopted as a filter, since the resonant vibration of the piezoelectric body 501 is utilized, a downsized and low-loss filter can be realized. When two thin film elastic wave resonators are connected in series and in parallel (FIG. 24(a)), a band-pass filter can be easily structured (FIG. 24(b)).
In reality, since the thin film elastic wave resonator is invariably fixed on the substrate, all of the longitudinal vibration, in the thickness direction, generated at a vibration section is not excited as main resonant vibration and a part of the vibration leaks to the substrate. This vibration leakage to the substrate (unnecessary vibration) means that a part of energy to be originally used in excitation of vibration inside of the piezoelectric body is treated as a loss. Therefore, the invention which reduces an energy loss is disclosed in patent document 2 or the like.
In addition, since in a communication device using a high-frequency band, noise is generated in a transmission path (wiring) on a substrate on which various electronic components are connected, a balance-type (differential-type) transmission path is employed as a countermeasure. Here, the balance-type transmission path is a parallel transmission path which handles two signals whose amplitudes are equal to each other and whose phases are opposite to each other. Accordingly, addition of a balance-unbalance conversion function into the thin film elastic wave resonator or a band-pass filter using the thin film elastic wave resonator is required. For the balance conversion-type thin film elastic wave resonator, a structure in which two thin film elastic wave resonators are disposed in an adjacent manner so as to share a piezoelectric body and propagation (coupling) of traverse-mode vibration generated in a vibration section is utilized is often adopted. It has been well known that since the thin film elastic wave resonator generally operates with resonance of a frequency at which a thickness is equal to a half-wave length, a phase difference between the upper electrode and the lower electrode is ideally 180 degrees, thereby realizing the balance conversion.
[Patent document 1] Japanese Laid-Open Patent Publication No. 60-68711
[Patent document 2] Japanese Patent No. 2644855